The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

But I never forgot the patients like Mikhey whom I had encountered during my neurology rotation as a medical student. In neurology, it seemed, there were so many questions left unresolved. Why did Mikhey laugh when poked? Why does the big toe go up when you stroke the outer border of the foot of a stroke patient? Why do patients with temporal lobe seizures believe they experience God and exhibit hypergraphia (incessant, uncontrollable writing)? Why do otherwise intelligent, perfectly lucid patients with damage to the right parietal lobe deny that their left arm belongs to them? Why does an emaciated anorexic with perfectly normal eyesight look in a mirror and claim she looks obese? And so, after years of specializing in vision, I returned to my first love: neurology. I surveyed the many unanswered questions of the field and decided to focus on a specific problem: phantom limbs. Little did I know that my research would yield unprecedented evidence of the amazing plasticity and adaptability of the human brain.

It had been known for over a century that when a patient loses an arm to amputation, she may continue to feel vividly the presence of that arm—as though the arm’s ghost were still lingering, haunting its former stump. There had been various attempts to explain this baffling phenomenon, ranging from flaky Freudian scenarios involving wish fulfillment to invocations of an immaterial soul. Not being satisfied with any of these explanations, I decided to tackle it from a neuroscience perspective.

I remember a patient named Victor on whom I conducted nearly a month of frenzied experiments. He came to see me because his left arm had been amputated below the elbow about three weeks prior to his visit. I first verified that there was nothing wrong with him neurologically: His brain was intact, his mind was normal. Based on a hunch I blindfolded him and started touching various parts of his body with a Q-tip, asking him to report what he felt, and where. His answers were all normal and correct until I started touching the left side of his face. Then something very odd happened.

He said, “Doctor, I feel that on my phantom hand. You’re touching my thumb.”

I used my knee hammer to stroke the lower part of his jaw. “How about now?” I asked.

“I feel a sharp object moving across the pinky to the palm,” he said.

By repeating this procedure I discovered that there was an entire map of the missing hand on his face. The map was surprisingly precise and consistent, with fingers clearly delineated (Figure 1.1). On one occasion I pressed a damp Q-tip against his cheek and sent a bead of water trickling down his face like a tear. He felt the water move down his cheek in the normal fashion, but claimed he could also feel the droplet trickling down the length of his phantom arm. Using his right index finger, he even traced the meandering path of the trickle through the empty air in front of his stump. Out of curiosity I asked him to elevate his stump and point the phantom upward toward the ceiling. To his astonishment he felt the next drop of water flowing up along the phantom, defying the law of gravity.

FIGURE 1.1 A patient with a phantom left arm. Touching different parts of his face evoked sensations in different parts of the phantom: P, pinky; T, thumb; B, ball of thumb; I, index finger.

Victor said he had never discovered this virtual hand on his face before, but as soon as he knew about it he found a way to put it to good use: Whenever his phantom palm itches—a frequent occurrence that used to drive him crazy—he says he can now relieve it by scratching the corresponding location on his face.

Why does all this happen? The answer, I realized, lies in the brain’s anatomy. The entire skin surface of the left side of the body is mapped onto a strip of cortex called the postcentral gyrus (see Figure Int.2 in the Introduction) running down the right side of the brain. This map is often illustrated with a cartoon of a man draped on the brain surface (Figure 1.2). Even though the map is accurate for the most part, some portions of it are scrambled with respect to the body’s actual layout. Notice how the map of the face is located next to the map of the hand instead of being near the neck where it “should” be. This provided the clue I was looking for.

Think of what happens when an arm is amputated. There is no longer an arm, but there is still a map of the arm in the brain. The job of this map, its raison d’être, is to represent its arm. The arm may be gone but the brain map, having nothing better to do, soldiers on. It keeps representing the arm, second by second, day after day. This map persistence explains the basic phantom limb phenomenon—why the felt presence of the limb persists long after the flesh-and-blood limb has been severed.